A capacitance type occupant sensor is, for example, disclosed in JP 2008-111809 A, which corresponds to U.S. 2008/0100425 A1. The capacitance type occupant sensor has an electrode installed inside a seat of a vehicle. The capacitance type sensor determines whether the seat is occupied by an occupant and a type of the occupant, such as an adult or a child seated on a child restraint system (CRS) (hereinafter, also referred to as a child safety seat) based on the change in capacitance between the electrode and a reference electrode provided by such as a vehicle body.
Specifically, the capacitance to be sensed varies according to a difference of a specific inductive capacity of an object to be sensed, which is located between the electrodes. The object to be sensed is discriminated based on the change of the capacitance. For example, the specific inductive capacity of air is approximately 1, and the specific inductive capacity of the CRS is approximately 2 to 5. Also, the specific inductive capacity of an adult is approximately 50. The change of the capacitance can be obtained from an impedance calculated based on current and voltage sensed between the electrodes.